Sorted by IM3

Sorted by IM3

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Sorted  by Intermodulation. Best receiver first. 

This is a measure of
how badly a close-spaced signal will trash your receiving with spurious signals
or splatter sounds, and you might blame it on the other guy thinking your
receiver is just great. Remember that even in my very quiet rural location
where I have to “beam” through the east coast to hear Europe, 80dB or
so IM3 is probably more than enough. The reason my Drakes are so good is because
they were built to duplex on the same band with my own transmitters running!!
Unless you are in a situation like that, anything over 80dB is likely wasted.

Most transmitters
are really the
source of problems.
Exceptions to
transmitters being a primary source of 
off or adjacent
channel interference occurs
if:

1.) You use a noise blanker. Most noise blankers kill receiver IM and
blocking performance.

2.) You don’t have good narrow IF filters.

3.) You use too much gain in the receiver front end.

4.) You have a receiver with a design problem (less than 80-85dB close-spaced
IM3 DR) . 

Beyond the above, most transmitters are the real problem. Look at transmitter
IM tests on SSB and read about keyclicks. 

Poor Designs

Poor designs include:

1.) Many different Yaesu’s without noise blanker mods. NO, changing the
diodes doesn’t do a thing! Been there, done that. The hissing noise comes from a
FET following the narrow filters, the IM comes from the silly Yaesu
noise-blanker design that hangs an FET right on a wide IF line with the
potential for that FET to be at full gain even when the NB is OFF!!!

2.) Early DSP filter only radios.
Many aren’t that good. If you have a DSP radio
without a
roofing filter that matches the mode, you
could have a poor receiver.
There are
exceptions, but they
are still not as
good as a better
narrow crystal or
mechanical filter
radio.

3.) Radios with spurious response problems. The JRC JST-245’s I’ve tested and
used all test good, but they have so many spurious synthesizer responses on 160
meters I’d never consider using one for serious weak signal work. I don’t know
how they are on other bands, but the
spurious responses
make them poor
performers on 160.

You can see how the
noise blanker FET in the
Yaesu hurts the performance.
The W8JI modified
Yaesu’s have the
noise blanker
transistor modified
to turn fully off.
The stock early R4C
uses a dual gate
MOSFET mixer that is
subject to overload,
and is one of the
worse receivers
manufactured.
10KHz or wider tests
are almost
meaningless. Almost anything is good at 10kHz, even some very poor
receivers.   Here’s how a few receivers stack up:

Note 1: The FT1000MP has the same inherent problem as the FT1000 and FT1000D.

A noise-blanker amplifier FET is left on even when the noise blanker is turned off with
front panel NB and NB-W switches.
This FET connects directly to the IF, and overloads when strong signals are
present. Unlike the FT1000 and FT1000D, the front panel noise blanker level
control affects the FT1000MP IMD performance. * is with the NB gain control fully
advanced, ** is with the NB gain control fully off. In ALL cases the NB
off-on switch is OFF!!

Note 1A: Yaesu copied the same basic NB from the FT1000D, and
so this radio also has a NB problem even when the NB is switched off. This test
was with NB gain menu set for minimum gain. The radio is worse when NB gain is
increased in the NB gain menu.   

Note 2: This is a MOSFET 2nd mixer R4C, aligned and in good shape but with no
changes. This receiver is almost useless for weak signal CW work of any kind
because of poor IM performance and filter blow-by.  

Note 3: This is a tube 2nd mixer R4C, aligned and in good shape but with no
changes. This receiver is almost useless for contesting or crowded bands because
of poor IM performance and bad filter blow-by. 

Note 4: This is the same receiver in note 2, but with the following changes:

1. Increased screen voltage on RF amplifier to 130 volts.
2. 6J6 first mixer with oscillator injection increased to 5 volts.
3. Diode double-balanced 2nd mixer with +10dBm injection.
4. 10dB of gain removed from first mixer and first IF and redistributed
   behind the narrow filter.
5. Sherwood Engineering 600Hz roofing filter.
6. New AGC system. After narrow filter only, no AGC on front end.

Note 5: Gutted and rebuilt with solid-state double balanced high-level
mixers. No vacuum tubes in signal path. Sherwood 600Hz roofing
filter. Experimental receiver for full-duplex operation on 160 meters. This
receiver allows me to hear weak DX while I am transmitting, as long as they are
a few kHz off frequency.